The general objectives of this proposal are to characterize by electron microscopy avian tumor virus RNA from transforming and non- transforming viruses and products of the viral replicase. We have developed methods for the study of single-stranded RNA's by electron microscopy, and we are applying these methods to RNA from transforming and non-transforming viruses to determine the composition and topology of virus 60-70S RNA and the subunits composing the viral RNA. We are also studying by electron microscopy the products of endogenous template, primer, and replicase to determine the number and location of initiation sites for the viral RNA-dependent DNA polymerase and the portion of the genome copied into RNA-DNA hybrids. This information will be used for further studies of viral nucleic acid replication.